Google and other leading search engines rank websites that are registered for long terms higher. A five or ten year domain registration represents a higher level of commitment to providing quality content. If your domain is up for renewal you may consider the five year term renewal. Besides you save money.
August 13, 2013 Update: At the time the original post was written in 2008, it was widely held by many top SEO firms that the domain registration term influenced rankings. Matt Cutts has stated that the quality of your site, and the quality of links pointing to your site influence ranking, and that the registration period for the domain is not the primary factor. So does it affect rankings? He didn't really say for sure.